1. Field of the Invention
This invention relates to a method and apparatus for use in the manufacture of non-woven fibrous material such as paper.
2. Description of the Prior Art
Paper and other non-woven fibrous materials are conventionally made by depositing a suspension of fibres in a liquid medium, usually water, onto a foraminous support, for example the wire of a Fourdrinier-type paper making machine, which allows the liquid medium to drain through while retaining most of the fibres on the wire in the form of a web.
A problem encountered with such methods is that the fibres in the suspension have an inherent tendency to form flocs, or clumps, which are difficult to disperse, and may remain in the formed web, imparting to it an uneven appearance and adversely affecting its useful properties. This problem is particularly serious when dealing with suspensions of relatively long fibres, or of synthetic fibres, or when using relatively high concentrations of fibre in the suspension, as may be the case if a heavy web is to be produced, or if the web is of inherently slow draining fibres, or if the speed of the paper making machine is high, or if it is important to reduce the costs of purifying the drained liquid medium.
The problem of flocculation of fibres can be mitigated by using very low concentrations of fibres in water, i.e. concentrations of the order of 10 mg/l to 100 mg/l instead of the more usual 1 g/l to 10 g/l, but this involves the handling of very large quantities of water, which in general is not commercially practicable.
It has also been proposed to overcome the problem of flocculation by producing even dispersions of fibres in high viscosity solutions of polymeric materials, but the use of such fibre suspensions on a paper making machine severely limits productivity, because of the inherent slow rate of drainage of such viscous suspending liquid media.
It has also been proposed to overcome the problem of flocculation by producing uniform dispersions of fibres in foams made from air dispersed in water in the presence of a foaming agent (see for example British Patent Specification No. 1,209,409). Such foams drain relatively easily, and do not restrict unduly the productivity of a paper making machine. It is thought that the advantageous rheology of such foams results from the presence of closely packed rigid bubbles which behave like a pseudo-plastic body, which restricts the movement of fibres, and so prevents their flocculation, but yields under the stresses involved in fibre dispersion and drainage. Relatively high concentrations of foaming agents in the suspension have so far been found necessary to impart sufficient stability to the foam to enable it to carry the dispersed fibres from the dispersing apparatus to the foraminous support. Such foaming agents are thought to interfere with the forces which bond conventional paper making fibres, and therefore their presence tends to reduce the density and strength of the paper product obtained. This problem can be overcome at least to some extent by using a greater degree of beating of the fibres, or by the addition of strengthening agents, but it would be advantageous if the amount of surfactant used could be reduced.
Another method of overcoming the problem of flocculation has been proposed which uses a paper making machine headbox slice in which is a very shallow (e.g. 0.5 to 3.0 mm) passage having a series of sharp bends therein. When a fibre dispersion is passed through the passage, the bends set up high intensity, small-scale turbulence in the dispersion which tends to break up any fibre flocs in the dispersion. The dispersion is then passed through a second very shallow passage and delivered directly on to a foraminous support, such as the wire of a paper making machine. The turbulence of the flow of the dispersion is said to decay in the second passage and the resistance to shear of the well dispersed mass of fibres at the high concentration of the dispersion tends to prevent flocculation, or re-flocculation, occuring. Dispersions having fibre concentrations of 30 to 40 g/l are said to be usable with the slice just described. However, such a slice has the disadvantage that it is only really suitable for making papers of a high substance, because the concentrated dispersion delivers excessive amounts of fibre to the wire even when the second passage is as shallow as 2-3 mm. Moreover, we have found that the turbulence set up is not completely effective in dispersing clumps of fibres, which may clog the shallow passage. Adventitious matter may also do this. A further problem is that there are serious engineering problems in making a slice having such shallow passages therein.